1. Field of the Invention
This application relates to an automatic gain repeater and more particularly to a gain repeater that includes controllable variable gain amplifiers in series circuit connection with a subscriber loop to maintain constant subscriber loop line loss independent of line lengths.
2. Description of the Prior Art
Automatic gain repeaters are known in the prior art. One type of repeater is described in U.S. Pat. No. 4,049,928 issued to Levi. Operation of the described repeater relies on determining loop resistance by applying a constant current in the subscriber loop. A deficiency of repeaters of this design resides in the fact that they require a well-regulated constant current source and a feedback circuit for voltage control of the gain. Feedback controllers of this type are of marginal stability and require sophisticated stabilizing filter networks. Monitoring of the current and control of the repeater amplifiers thus requires addition of circuitry which ultimately adds to both the cost and complexity of the unit. Another deficiency is the inability of this design to differentiate between changes in loop current due to loop length as opposed to those due to changes in the battery feed voltage.
Another such repeater is described in U.S. Pat. No. 4,056,688 issued to Stiefel. Operation of the described repeater relies, again, upon the measurement of current in the subscriber loop to determine its resistance and the amount of gain required to maintain a constant transmission signal level.
A feature of the described device is that the gain change occurs in quantized discrete steps. Thus, noticeable volume differentials occur between users where a gain change occurs intermediate between the two user stations.
Another such repeater is described in U.S. Pat. No. 4,061,883 to Chambers. This approach requires the simultaneous measurement of both current and voltage to determine the loop resistance. This is in contrast to the present invention which requires no direct measurement of any line parameters. Chambers also requires the use of costly and complex four quadrant multiplier circuits to achieve its automatic gain control. Considerable power is also required by this design. Such power could better be used to provide current boost to the line as is done in the present invention. The device is also characterized by discrete gain changes with the effects as previously described above.